This invention relates to apparatus for converting such physical quantities as pressure, displacement, tension, temperature, light quantity, etc., into variations in electric current.
Such converting apparatus are used to detect fluid pressure in various manufacturing processes, for example, and the detected pressure is converted into a variation in current. Then the detected result is transmitted to a remote receiving device. Heretobefore, physical quantity converting apparatus having circuit constructions as shown in FIGS. 1 and 2 have been used.
More particularly, the physical quantity converting apparatus comprises a bridge circuit constituted by resistors 101, 102, 104, 105 and a variable resistor 103 acting as a physical quantity detecting element and a Zener diode 106 for applying a stabilized voltage across the bridge circuit. The juncture between the resistor 101 and the variable resistor 103 and that between the resistors 102 and 105 are connected respectively to the non-inverting and inverting inputs of a differential amplifier 107. Thus, when the resistance value of the variable resistor 103 varies, the potential difference between the two inputs of the differential amplifier 107 is amplified and its output is applied to the base electrode of a transistor 109 for varying the current flowing through a load resistor 114 supplied from a DC source 113 via line terminals 111 and 112, resistors 104 and 110 and the collector emitter path of a transistor 109.
However, as the resistance value of the variable resistor 103 increases the current flowing through the transistor 109 and resistor 110 is fed back to the line terminal 112 so that the polarity of the terminal voltage of the resistor 104 would be reversed with respect to that of the terminal voltage of the variable resistor 103. As a consequence a negative feedback circuit is formed through the differential amplifier so that the bridge circuit would be controlled such that the potential difference between two inputs of the amplifier 107 would be decreased to zero, and the bridge circuit becomes a balanced state. At this time, the current flowing through line terminals 111 and 112 will have a value proportional to the resistance value of the variable resistor 103.
A constant voltage circuit 108 is provided for supplying predetermined source voltages to the differential amplifier 107 and the Zener diode 106.
With the construction shown in FIG. 1, however, only an element that can pass current can be used as the element for detecting a physical quantity and it is impossible to use a variable capacitance element or the like as the detecting element.
FIG. 2 is a connection diagram showing a prior art physical quantity detecting apparatus utilizing a variable capacitance element, for example a differential capacitance type difference voltage detector as the detecting element. In this circuit, an oscillator 21 whose output amplitude is controllable is used as a source which is controlled by a differential amplifier 202 acting as a source control circuit. The output of the oscillator 201 is applied to a pair of variable capacitance elements 203 and 204 which differentially varies the capacitances.
Consequently, currents proportional to the frequency and amplitude of the output of the oscillator 201 and to the capacitances of the variable capacitance elements 203 and 204 flow through these capacitance elements. These currents are rectified by diodes 205 through 208. A positive half-wave current flows through a low pass filter constituted by capacitors 209, 211, 214 and resistors 210, 212 and 215, while a negative half-wave current flows through another low pass filter constituted by a resistor 216 and a capacitor 217. Consequently, a terminal voltage proportional to the capcitance elment 203 appears across the resistor 210, whereas a terminal voltage proportional to the capacitance of the variable capacitance element 204 appears across the resistor 212.
A terminal voltage proportional to the sum of the capacitances of the variable capacitance elements 203 and 204 appears across the resistor 215, and this terminal voltage is compared with a reference voltage 213 in the form of a battery by the differential ampifier 202. Since the output amplitude of the oscillator 201 is controlled by the output of the differential amplifier, the output amplitude of the oscillator 201 would be maintained in such a state in which the terminal voltage of the resistor 215 would be maintained at a constant value thus preventing the terminal voltages of the resistors 210 and 212 from being varied due to the variation in the output amplitude of the oscillator 201.
The terminal voltages of the resistors 210 and 212 are applied to both inputs of the differntial amplifier 107 via input resistors 219 and 220 respectively so as to control the transistor 109 in accordance with the difference in the voltages applied to the both inputs, whereby current proportional to the difference between the capacitances of the variable capacitance elements 203 and 204 flows through resistor 110, a potentiometer resistor 223 and line terminals 111 and 112.
It is desirable to maintain the value of the current flowing through line terminals 111 and 112 in a range of 4 to 20 mA regulated as a standard in the field of industrial measurement. To this end the potentiometer resistor 218 is connected across a Zener diode 106 and a negative terminal of the resistor 216, and a predetermined voltage produced by the potentiometer resistor 218 is applied to the inverting input of a differential amplifier 107 via an input resistor 221 so as to set the current to 4 mA within a standard range. On the other hand a voltage of an opposite polarity and produced by the potentiometer resistor 223 is negatively fed back to the non-inverting input of the differential transformer 107 via an input resistor 222, and the amount of feed back is adjusted with potentiometer resistor 223 to obtain a maximum current of 20 mA.
Although in the circuit shown in FIG. 2, the negative feedback loop through the differential amplifier 202 and the negative feedback loop through the differential amplifier 107 are formed independently. As no feedback loop is formed for the entire circuit, when noise enters into the input side of the differential amplifier 107 to cause some kind of external disturbance, the disturbance would be superposed upon the current flowing through the line terminals 111 and 112 thus causing an error in the result of measurement with the variable capacitance elements 203 and 204.